ftp: Add LoginCellProvider for FTP

Adding a dedicating FTP provider for LoginCellFactory
allows the shared executor to be bound to the lifetime
of the LoginManager.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6310/